The successful picture of strongly-interacting particles, hadrons, as being composites built from quarks and antiquarks is justified within Quantum Chromodynamics, the quantum field theory describing the interactions of quarks and gluons.

The strong coupling of quarks to the gluonic field explains many observed phenomena in hadron physics, but the impact of the strong coupling of the gluonic field to itself is not directly visible in the current experimental spectrum of hadrons. We might expect hadrons in which the glue is excited, but we have not definitively observed them to-date.

I will present results of recent numerical calculations of QCD that indicate the role excited glue should play in the spectrum of hadrons, and the consequent exotic particles that can be searched for in current and near-future experiments.